The present invention relates generally to a hybrid electric vehicle (HEV), and specifically to a strategy to provide vehicle creep and hill holding similar to a conventional internal combustion vehicle with an automatic transmission.
The need to reduce fossil fuel consumption and emissions in automobiles and other vehicles predominantly powered by internal combustion engines (ICEs) is well known. Vehicles powered by electric motors attempt to address these needs. Another alternative solution is to combine a smaller ICE with electric motors into one vehicle. Such vehicles combine the advantages of an ICE vehicle and an electric vehicle and are typically called hybrid electric vehicles (HEVs). See generally, U.S. Pat. No. 5,343,970 to Severinsky.
The HEV is described in a variety of configurations. Many HEV patents disclose systems where a driver is required to select between electric and internal combustion operation. In other configurations, the electric motor drives one set of wheels and the ICE drives a different set.
Other, more useful, configurations have developed. For example, a series hybrid electric vehicle (SHEV) configuration is a vehicle with an engine (most typically an ICE) connected to an electric motor called a generator. The generator, in turn, provides electricity to a battery and another electric motor, called a traction motor. In the SHEV, the traction motor is the sole source of wheel torque. There is no mechanical connection between the engine and the drive wheels. A parallel hybrid electrical vertical (PHEV) configuration has an engine (most typically an ICE) and an electric motor that work together in varying degrees to provide the necessary wheel torque to drive the vehicle. Additionally, in the PHEV configuration, the motor can be used as a generator to charge the battery from the power produced by the ICE.
A parallel/series hybrid electric vehicle (PSHEV) has characteristics of both PHEV and SHEV configurations and is sometimes referred to as a powersplit configuration. In one of several types of PSHEV configurations, the ICE is mechanically coupled to two electric motors in a planetary gear-set transaxle. A first electric motor, the generator, is connected to a sun gear. The ICE is connected to a carrier gear. A second electric motor, a traction motor, is connected to a ring (output) gear via additional gearing in a transaxle. Engine torque can power the generator to charge the battery. The generator can also contribute to the necessary wheel (output shaft) torque if the system has a one-way clutch. The traction motor is used to contribute wheel torque and to recover braking energy to charge the battery. In this configuration, the generator can selectively provide a reaction torque that may be used to control engine speed. In fact, the engine, generator motor and traction motor can provide a continuous variable transmission (CVT) effect. Further, the HEV presents an opportunity to better control engine idle speed over conventional vehicles by using the generator to control engine speed.
The desirability of combining an ICE with electric motors is clear. There is great potential for reducing vehicle fuel consumption and emissions with no appreciable loss of vehicle performance or drivability. The HEV allows the use of smaller engines, regenerative braking, electric boost, and even operating the vehicle with the engine shutdown. Nevertheless, new ways must be developed to optimize the HEV's potential benefits.
A successful HEV implementation should consider that drivability and performance of the vehicle meet driver expectations of a comparable conventional ICE powered vehicle.
One such area of HEV development is providing vehicle creep and hill holding comparable to a conventional ICE vehicle with an automatic transmission. A HEV controller to meet this expectation needs to be developed.
HEV controllers are known in the prior art. Severinsky describes a simplistic HEV control unit. Other patents refer to creep functions for an HEV, but only generally and only as part of an overall configuration. U.S. Pat. No. 5,771,478 to Tsukamoto et al. describes current flows through the generator/motor, making it possible to generate a creep force similar to that of a conventional torque converter. U.S. Pat. No. 5,801,499 to Tsuzuki et al., has a “no-creep” mode to prevent vehicle movement. U.S. Pat. No. 5,887,670 to Tabata et al. and U.S. Pat. No. 5,984,034 to Morisawa et al. have creep calculations in various drive modes to mimic conventional engine creep at idle speeds. And, U.S. Pat. No. 6,093,974 to Tabata et al. mimics the creep force in electric mode by maintaining braking pressure even after the brake pedal is released.
Unfortunately, none of the known prior art appear to have the strategy of the present invention combining powertrain mode and motor temperature to provide hill holding and vehicle creep comparable to a conventional ICE vehicle with an automatic transmission while optimizing total powertrain system efficiency and performance in various operating states. This would include a controller to provide this feature even when an engine is not even running.